1. Field of the Invention
This invention relates generally to a fuel cell system and, more particularly, to a fuel cell system employing a recuperative heat exchanger for providing additional cooling of the charge air and the fuel cell stack in the system.
2. Discussion of the Related Art
Hydrogen is a very attractive source of fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives a hydrogen gas and the cathode receives oxygen. The hydrogen gas is ionized in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen ions react with the oxygen and the electrons in the cathode to generate water as a by-product. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform electrical work, before being sent to the cathode. The work acts to operate the vehicle. Many fuels cells are combined in a stack to generate the desired power.
Proton exchange membrane (PEM) fuel cells are a popular fuel cell for vehicles because they provide high power densities by high system efficiencies. In a PEM fuel cell, hydrogen (H2) is the anode reactant, i.e., fuel, and oxygen is the cathode reactant, i.e., oxidant. The cathode reactant can be either pure oxygen (O2) or air (a mixture of mainly O2 and N2). The electrolytes are solid polymer electrolytes typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode and cathode are typically comprised of finely divided catalytic particles, which are often supported on carbon particles and mixed with a proton conductive resin.
FIG. 1 is a general schematic plan view of a known PEM fuel cell system 10 of the type discussed above. The fuel cell system 10 includes a conventional fuel cell stack 12 having a plurality of fuel cells 14 electrically coupled in series. Each of the fuel cells 14 includes a cathode and an anode. The fuel cells 14 receive an anode hydrogen gas from a suitable source on a line 18 and a cathode charge gas (compressed air) on a line 20 to provide the chemical reaction that generates output power 22 to drive the vehicle. A series of cooling channels 24, represented in the drawings as a heat exchanger, running through the stack 12 removes heat therefrom generated by the chemical reactions in the fuel cells 14.
Anode exhaust gas is, for example, exhausted from the stack 12 on line 28 through a back pressure valve (BPV) 26. Pressurized cathode exhaust gas is exhausted from the stack 12 on line 30 at the temperature of the fuel cell stack 12, and makes up the major portion of the system exhaust. Water is a by-product of the cathode exhaust, but it would be problematic to release liquid water into the environment. Therefore, the cathode exhaust gas is applied to a liquid separator 32 that separates liquid water therefrom, and provides the separated exhaust gas on line 34 and liquid water on line 38. The separated cathode exhaust gas is output to atmosphere through a BPV 36. The liquid water on the line 38 can be provided to other system elements that may use water for cooling and the like.
Ambient charge air on line 42 is applied to a compressor 44 to compress the volume of the air to provide the cathode gas at the fuel cell operating pressure. The compressor 44 is powered by an electrical motor 46 through an output shaft 48. The compressor 44 heats the charge air as it is compressed. The compressed and heated air is sent through a suitable charge air cooler (CAC) or heat exchanger 52 on line 50, where it is cooled. The waste heat of the compressor 44 is the thermal load of the heat exchanger 52. The cooled charge air on the line 50 is then sent to a humidification device 54 where it is mixed with water vapor. Water vapor needs to be mixed with the charge air so that there is moisture for the electrolyte between the anode and cathode in the fuel cells 14 to provide the necessary conductivity. The compressed and humidified charge air is then applied to the stack 12 on the line 20.
A coolant loop 58 provides a cooling fluid, such as a water/glycol mixture, to the cooling channels 24 and the heat exchanger 52. The cooling fluid is forced through the loop 58 by a coolant pump 56. The heated cooling fluid is delivered by the loop 58 to a radiator fan module (RFM) 62 to remove the heat therefrom. In one embodiment, the temperature of the charge air on the line 50 at the output of the compressor 40 is in the range of ambient to 200° C., and the temperature of the charge air on the line 20 provided to the stack 12 is in the range of 60°-80° C. A fan 64 forces air through the RFM 62 to cool the heated fluid from the cooling channels 24 and the heat exchanger 52. The cooling fluid is then sent back through the coolant loop 58, first to the heat exchanger 52 to cool the compressed charge air on the line 50 and then to the stack 12, where it flows through the cooling channels 24.
In current fuel cell system designs, the RFM 62 is the typical radiator employed in conventional vehicles having an internal combustion engines. However, the operating temperature of an internal combustion engine is greater than the operating temperature of the fuel cell system 10, and thus fuel cell systems need to be cooled to a lower temperature level than internal combustion engines. Therefore, current RFMs used for internal combustion engines would not provide sufficient heat exchange area and air mass flowing therethrough to provide enough cooling for the system 10. The total system off heat (including the heat from the heat exchanger 52) is a critical limiting factor in the design of the system 10 and has significant impact on the system layout and design. It would be desirable to provide an additional technique for removing heat from the system 10 so that the known RFMs can be employed within the vehicle.